Why would you limit your world to one of those settings... they all existed.. maybe not in the same place or the same time.. but its easy enough to do what I do. the world is about 900 AD and all the best bits of history, legend, faith, and fantasy (for our gaming purposes) all exist... Norse is up North (some place), Celtic is English Isles and France and the Slavs are down the river into "USSR".

Guys, In history the "vikings" went to all these places, the Danes into Normandy (France), The Norwegians over to Ireland (the Saxons were sort of tough), and the Swedes down the rivers into Russian and slavic areas... they were the Rus... thus the name... the key point, Romans did not conquer the Greek city states and The Jews and if you want them the Christians remain a small local sect... so that the Romans do not conquer and Christianity does not supplant... works for me and seems like it will work for you all... just a thought.

Wow, Another Natural One! You guys are a sink hole for luck. Stay away from my dice.

I know FG has a C&C ruleset, but does it let you modify the rules? For instance, can you add the classes from the Codices, or modify for house rules, etc?

You can create your own classes, make them in a module and then they can available across multiple campaigns. Depending on your house rules, simple changes should be easy to implement, more complex ones might require an extension to be created.